Calcium plays an indispensable role in cell permeability, the formation of bones and teeth, blood coagulation, transmission of nerve impulse, and normal muscle contraction. The concentration of calcium ions in the blood is, along with calcitrol and calcitonin, regulated mainly by parathyroid hormone (PTH). Although calcium intake and excretion may vary, PTH serves through a feedback mechanism to maintain a steady concentration of calcium in cells and surrounding fluids. When serum calcium lowers, the parathyroid glands secrete PTH, affecting the release of stored calcium. When serum calcium increases, stored calcium release is retarded through lowered secretions of PTH.
The complete form of human PTH, (hPTH), is a unique 84 amino acid peptide (SEQ ID NO: 1), as is shown in FIG. 1. Researchers have found that this peptide has an anabolic effect on bone that involves a domain for protein kinase C activation (amino acid residues 28 to 34) as well as a domain for adenylate cyclase activation (amino acid residues 1 to 7). However, various catabolic forms of clipped or fragmented PTH peptides also are found in circulation, most likely formed by intraglandular or peripheral metabolism. For example, whole PTH can be cleaved between amino acids 34 and 35 to produce a (1-34) PTH N-terminal fragment and a (35-84) PTH C-terminal fragment. Likewise, clipping can occur between either amino acids 36 and 37 or 37 and 38. Recently, a large PTH fragment referred to as “non-(1-84) PTH” has been disclosed which is clipped closer to the N-terminal end of PTH. (See LePage, R., et al., Clin. Chem. 44: 805-810 (1998).
The clinical need for accurate measurement of PTH is well demonstrated. Serum PTH level is one of the most important index for patients with the following diseases: familial hypocalciuric hypercalcemia; multiple endocrine neoplasia types I and II; osteoporosis; Paget's bone disease; primary hyperparathyroidism—caused by primary hyperplasia or adenoma of the parathyroid glands; pseudohypoparathyroidism; and renal failure, which can cause secondary hyperparathyroidism.
PTH plays a role in the course of disease in a patient with chronic renal failure. Renal osteodystrophy (RO) is a complex skeletal disease comprising osteitis fibrosa cystica (caused by PTH excess), osteomalacia-unmineralized bone matrix (caused by vitamin D deficiency), extraskeletal calcification/ossification (caused by abnormal calcium and phosphorus metabolism), and adynamic bone disease (contributed to by PTH suppression). Chronic renal failure patients can develop RO. Failing kidneys increase serum phosphorus (hyperphosphoremia) and decrease 1,25-dihydroxyvitamin D (1,25-D) production by the kidney. The former results in secondary hyperparathyroidism from decreased gastrointestinal calcium absorption and osteitis fibrosa cystica from increased PTH in response to an increase in serum phosphorus. The later causes hypocalcemia and osteomalacia. With the onset of secondary hyperparathyroidism, the parathyroid gland becomes less responsive to its hormonal regulators because of decreased expression of its calcium and vitamin D receptors. Serum calcium drops. RO can lead to digital gangrene, bone pain, bone fractures, and muscle weakness.
Determining circulating biologically active PTH levels in humans has been challenging. One major problem is that PTH is found at low levels, normally 10 pg/mL to 40 pg/mL (i.e., 1 pmol/L to 4 pmol/L). Coupled with extremely low circulating levels is the problem of the heterogeneity of PTH and its many circulating fragments. In many cases, immunoassays have faced substantial and significant interference from circulating PTH fragments. For example, some commercially available PTH kits have almost 100% cross-reactivity with the non-(1-84) PTH fragment. See the LePage article supra.
PTH immunoassays have varied over the years. One early approach is a double antibody precipitation immunoassay found in U.S. Pat. No. 4,369,138, issued to Arnold W. Lindall et alia. A first antibody has a high affinity for a (65-84) PTH fragment. A radioactive labeled (65-84) PTH peptide is added to the sample with the first antibody to compete for the unlabeled peptide. A second antibody is added which binds to any first antibody and radioactive labeled PTH fragment complex, thereby forming a precipitate. Both precipitate and supernatant can be measured for radioactive activity, and PTH levels can be calculated therefrom.
In an effort to overcome PTH fragment interference, immunoradiometric two-site assays for intact PTH (I-PTH) have been introduced, such as Allegro® Intact PTH assay by the Nichols Institute of San Juan Capistrano, Calif. In one version, a capture antibody specifically binds to the C-terminal portion of hPTH while a labeled antibody specifically binds to the N-terminal portion of the captured hPTH. In another, two monoclonal antibodies were used, both of which attached to the N-terminal portion of hPTH. (For the purposes of the present disclosure, the complete form of human PTH is referred to as “whole PTH” or “wPTH” as distinguished from “intact PTH” or “I-PTH” which can include not only wPTH, but also a large PTH fragment cleaved about amino acids 5 to 8.) Unfortunately, these assays have problems in that they measure but do not discriminate between w-PTH and I-PTH. This inability comes to the fore in hyperparathyroid patients and renal failure patients who have significant endogenous concentrations of large, non-whole PTH fragments.
Recently, researchers have made a specific binding assay directed to the large N-terminal PTH fragments. See Gao, P., et al., Clinica Chimica Acta 245: 39-59 (1996). This immunochemiluminometric assay uses two monoclonal antibodies to detect N-terminal (1-34) PTH fragments but not mid-portion PTH fragments or C-terminal PTH fragments. A key factor in the design of these assays is to eliminate any reaction with C-terminal PTH fragments.
Nevertheless, specific whole PTH assays have not been able to measure whole PTH at physiological levels. See, e.g., Magerlein, M., et al., Drug Res. 48:197-204 (1998). The present invention is intended to meet these and other needs in the art.
An important discovery leading to the present invention is that adynamic bone loses its capacity to buffer calcium and phosphate as the bones are shut down. In subjects afflicted with such conditions, they are unable to effectively buffer calcium as it enters their bodies through their diet. This calcium enters the blood stream and is thereafter shuttled to the soft tissues. The parathyroid gland is particularly subject to, and detrimentally affected by, this influx of calcium and thereby produces PTH fragments rather than, or in addition to, the active form of PTH. Accordingly, in subjects with adynamic bone, the concentration and production of PTH fragments is increased. In light of this and other related information, the measurement of PTH fragment levels, and particularly in conjunction with the measurement of whole PTH, can be used effectively to differentiate subjects having adynamic bone versus those having normal bone and high bone turnover rates.
There is a tremendous need to be able to non invasively separate the dialysis patients with ADN from those suffering from high bone turnover to avoid over treatment of ADN dialysis patients. Over treatment of dialysis patients with ADN is a frequent occurrence under presently utilized methods. For example, package inserts that proscribe the use of Zemplar® and Calcijex® (Abbott Laboratories), for example, are being used to treat thousands of dialysis patients that stand a great risk of over treatment under the proscribed protocols that do not account for circulating total PTH fragment levels. The present invention addresses these and other need in the art.